Generally, a distributed amplifier comprises a plurality of basic amplifying cells, mounted in series with at least one common drain line and one common grid line. Each basic amplifying cell comprises at least one field effect transistor which is common-source mounted and connected to the common drain line by its drain and to the common grid line by its grid.
The correct functioning of a field effect transistor requires the application of a first d.c. biasing voltage to its drain, on the one hand, and the application of a second d.c. biasing voltage to its grid on the other hand. In a distributed amplifier, the common drain line receives the first biasing voltage whilst the common grid line receives the second biasing voltage.
Sometimes, layouts use basic amplifying cells mounted in series with two common grid lines and one common drain line, each comprising several field effect transistors.
The basic requirements for the correct functioning of a distributed amplifier for wide band hyperfrequency signals relate to the filtering of the biasing circuits. These biasing circuits must in fact allow the d.c. biasing currents or voltages to pass but isolate the hyperfrequency power.
Most often, this filtering is carried out through the terminal resistors of the common grid and drain lines which can be coupled in series with a decoupling capacitor.
The filtering of the biasing of the common grid lines through their terminal resistor coupled in series with a decoupling capacitor is satisfactory insofar as the d.c. grid current is low and no voltage drop appears in the terminal grid resistors.
On the other hand, the filtering of the biasing of the common drain lines through their terminal resistor coupled in series with a decoupling capacitor is not suitable insofar as the d.c. drain current is high (for example 500 milliamperes for a distributed power amplifier of the order of 1 watt), which results in biasing voltage drops of the order of 25 volts for terminal drain resistors of value of the order of 50 ohms, and in consequence in a rise in the biasing voltage up to values not usually used in the field of integrated circuits.
The other disadvantages of the filtering of the biasing circuits through terminal grid or drain resistors are the large energy losses which occur around the said terminal resistors and the use of wide conductors occupying a lot of space on the semiconducting disc and which are necessary to bear the totality of the d.c. drain biasing current crossing the section of the common drain line which is connected up to the terminal drain resistor.
The filtering of the biasing circuits may also be carried out through reactive elements possibly coupled in parallel through a capacitor connected to earth. This type of filtering is satisfactory in the case of hybrid integrated circuits or discretely mounted field effect transistors.
On the other hand, in microwave monolithic integrated circuit technology, reactive elements are not suitable either when they are constructed in in-chip integrated form, or when they are constructed outside the said chip.
In fact, the filtering of the biasing circuits through reactive elements has the disadvantage of downgrading the performance levels of the distributed amplifier, in particular its impedance matching insofar as the reactive filtering elements may exhibit a disturbing natural resonance phenomenon, more particularly when a wide band distributed amplifier having a band of the order of 40 gigahertz is concerned. Moreover, these elements should, in the case of a power amplifier, bear a biasing current which is often incompatible with the size of the conducting wires.
When the reactive elements are mounted in external fashion on the integrated circuit, their dimensions lead, at least in their current construction, to a large spatial requirement which is very awkward in the construction of a microwave monolithic integrated circuit. Furthermore, their layout on the integrated circuit requires additional external connections as well as soldered joints whose implementation is difficult and sometimes awkward on a monolithic integrated circuit.
In known circuits other than power amplifiers, the filtering of the biasing circuits of the field effect transistors is ensured by an additional field effect transistor working in saturated mode. Nevertheless, the application of this type of filtering to the biasing circuits of a power amplifier is not possible.